Mechanism of photocatalysis

When photocatalyst titanium dioxide (TiO2) absorbs Ultraviolet (UV)
radiation from sunlight or illuminated light source (fluorescent
lamps), it will produce pairs of electrons and holes. The electron of
the valence band of titanium dioxide becomes excited when illuminated
by light. The excess energy of this excited electron promoted the
electron to the conduction band of titanium dioxide therefore creating
the negative-electron (e-) and positive-hole (h+) pair. This stage is
referred as the semiconductor's 'photo-excitation' state. The energy
difference between the valence band and the conduction band is known as
the 'Band Gap'. Wavelength of the light necessary for photo-excitation
is:

1240 (Planck's constant, h) / 3.2 eV (band gap energy) = 388 nm

The positive-hole of titanium dioxide breaks apart the water
molecule to form hydrogen gas and hydroxyl radical. The
negative-electron reacts with oxygen molecule to form super oxide
anion. This cycle continues when light is available.